The development of microminiature mechanical devices has advanced generally by use of a technique known as micromachining or microfabrication. See for instance, the discussion of microfabrication of mechanical devices by Angell et al. in "Silicon Micromechanical Devices," Scientific American, (April 1983), pp. 44-55.
A fundamental requirement of a micromechanical actuator (hereinafter, microactuator) is that some mechanical actuation means must be provided. A further requirement is that the actuation means must provide sufficient force for reliable actuation. For example, a microminiature device may comprise a valve used to control the flow of a carrier gas through a capillary column in a gas chromatograph. A microactuator may be required to open or close a fluid passage in the valve by displacing a moveable member (typically a moveable membrane, diaphragm, or boss) against a pressure of up to 200 pounds per square inch (1375 kilopascals), through a distance of as much as 100 micrometers.
Typically, electrical power from an external source is provided to the microactuator, which employs one of various techniques to convert the applied power to an actuating force. Often the applied electrical power is converted in part or whole to thermal power, and such microactuators can be considered as being thermally-actuated.
As disclosed in U.S. Pat. No. 5,058,856, an array of micromachined bimetallic legs has been employed to provide a thermal actuating force in a microminiature valve. The microminiature valve includes an actuator having radially spaced, layered spider legs, with each leg having first and second layers of materials having substantially different coefficients of thermal expansion. The legs include heating elements and are fixed at one end to allow radial compliance as selected heating of the legs causes flexure. Below the legs is a semiconductor substrate having a valve seat that defines a flow orifice. The actuator face is aligned with the valve seat. Flexure of the legs displaces the actuator face relative to the valve seat, thereby controlling fluid flow through the flow orifice.
However, the design and operation of such a valve is subject to a complex group of thermal, mechanical, and pneumatic constraints. Proportional control of gas flow at a wide range of supply gas pressures (from zero to 200 psi) and a wide range of flow rates (0.1-1000 standard cubic centimeters per minute (sccm)) requires significant actuation force and adequate stiffness in the mechanical structure. Further, the increase or decrease in the flow rate that occurs as the actuator face respectively moves to and from the orifice must be wellcontrolled.
For example, if the microminiature valve is normally closed when no power is applied, and if the thermal resistance from the actuator to its surroundings is low, the valve will require a relatively large amount of power to open, but will cool rapidly when power is removed and so will close rapidly. If the thermal resistance from the microminiature valve to its surroundings is high, the microminiature valve will require less power for to open, but will cool more slowly, and so will be slower to close.
In particular, FIG. 1 illustrates the measured response of flow rates of Helium to voltages applied to a conventional valve operating at supply pressures of 50 and 100 psi. Upon comparison of the response curves A and A' with curves B and B', the measured response indicates a hysteresis condition because more power is required to open the valve than to hold the valve open. A thermal hysteresis loop is evident as movement of the actuator face is initially subject to restraint due to the high supply pressure, then lifts from the valve seat when a threshold of substantial applied power is exceeded. The abrupt change causes the flow rate to increase at an abrupt and very high rate.
We have discovered that such abrupt action occurs because the separation of the actuator face from the seat allows the thermal conductance between the actuator face and valve seat to decrease rapidly and this decreased conductance causes the actuator to warm rapidly at an essentially constant input power. As a result, the valve opens in a fashion that is not easily controlled, as indicated by the nearly vertical slopes of the low flow rate portions of curves A and A'. In other words, the actuator will "snap" to an open position instead of moving gradually. Similarly the valve can "snap" close when the actuator approaches the valve seat from an open position. The effect is especially severe when the actuator face is constrained to move in an "irrotational" fashion, that is, when the actuator face maintains a parallel relationship with the valve seat while moving along an axis that is perpendicular to the valve seat, as taught in U.S. Pat. No. 5,069,419.
Accordingly, there is a need for a thermally-actuated microactuator, and in particular a thermally-actuated microminiature valve, which efficiently produces a controlled, gradual movement throughout the entirety of a its range of displacement when operated in the conditions described above.
FIG. 2 illustrates the displacement of an actuator in a typical thermally-actuated valve that is designed to exhibit irrotational actuating motion in response to an applied power. As indicated, in comparing the power required to maintain the actuator at positions proximate or distant from the valve seat, one may observe that much more power is required to maintain the actuator at a position proximate to the valve seat. However, prior art approaches have not sufficiently addressed this power loss during operation of a microactuated valve at low flow rates.
Accordingly, it is also desirable to minimize the power consumed by a microactuator subject to the above-described low-flow/high supply pressure conditions, and especially to reduce the power consumed by a microminiature valve where fast actuation is not critical. Accordingly, there is a need in thermally-actuated microactuators for improved efficiency of thermal actuation.